It has long been a goal in the semiconductor industry to produce PNP type transistors which have electrical characteristics approximately equivalent to NPN transistors and to provide complementary semiconductor structures having both NPN and PNP transistors. It is well known, however, that the gain and frequency response of PNP transistors is substantially less than that of NPN transistors. It has been believed that a significant advance in producing PNP transistors with high performance characteristics could be accomplished by providing a shallow P-doped emitter region with controlled doping concentration which is free from structural damage. By shallow emitter region is meant an emitter region having a depth from the silicon surface of less than about 3000 .ANG.. Numerous problems have been encountered in producing shallow P-doped emitter regions. For example, ion implantation of high dosages of boron into the surface of monocrystalline silicon provides a very high density of dislocation loops which extend about 500 .ANG. below the implant surface. With increasing depth below the implant surface, these loops coagulate into very dense tangles of dislocations which extend down to about 2000 .ANG.. Attempts to anneal these dislocations by heat treatment have been ineffectual. After annealing heat treatment a further layer of stacking faults is found to exist at a depth of from about 2000-5000 .ANG.. The stacking faults occupy a volume of the monocrystalline silicon which had not been implanted but are created by boron drive-in diffusion during the annealing heat treatment.
Attempts to produce P-type emitter regions by ion implantation with lower concentrations of boron are also ineffectual. The lower dosages result in the provision of concentrations of doping ions which are unacceptable to provide the required electrical properties. If the dosage is increased, the dislocation effects discussed above are incurred and the dislocations can not be cured by annealing.
A still further problem exists in that the diffusion rate of boron into the silicon structure is very fast. Any attempts to anneal the dislocation damage caused by providing the proper concentration of boron results in a drive-in depth of the diffused boron ions which is unacceptable. Accordingly, PNP transistors have been produced by methods other than ion implantation in the emitter region.
Similar problems have not existed in respect to the manufacture of NPN transistors. Ion implantation damage caused during implantation of N-type doping ions is more easily cured by annealing. While some dislocation faults, basically at the surface of the silicon, remain after annealing silicon implanted with N-type doping ions, transistor structures produced by ion implantation of N-type doping ions in the emitter region have been acceptable both as to depth of the emitter region and number and extent of dislocation faults. Nevertheless, while ion implanted N-type emitters have been generally accepted, numerous methods have been designed to improve this type of emitter. U.S. Pat. No. 3,460,007 to Scott, for example, describes a method for forming a P-N junction wherein N-doped polycrystalline silicon is deposited in-situ on the surface of monocrystalline silicon. The structure is subsequently heated to drive the N-type conductivity ion into the surface of the monocrystalline silicon to form an emitter region. Certain advantages are claimed for an N-doped emitter region formed in this manner.
An article by Graul et al., IEEE Journal of Solid State Circuits, Vol. SC-11, No. 4, August 1976, pp. 491-493 describes a method for forming an emitter region for an NPN transistor. In the method, an undoped polysilicon layer is deposited on the surface of a monocrystalline silicon surface. The polysilicon layer is implanted with an N-type doping ion, such as arsenic. The arsenic is then driven into the surface of the monocrystalline silicon to form an emitter region. Better emitter efficiency and higher current carrying capability are indicated to be provided by this method for forming an N-doped emitter region in an NPN transistor.
Because of differences in the physical characteristics between N-type doping ions and P-type doping ions, it has been believed that the diffusion of the doping ion from polysilicon is not a suitable method for the formation of P-type emitter regions. In the case of the in-situ doped polysilicon, it is not possible to obtain a high enough concentration of P-type doping ion to provide the emitter region with a suitable concentration level of P-type dopant. A further problem is that the drive-in characteristics of P-type doping ions were believed to be unsuitable for diffusion to provide a shallow emitter. It was also believed that ion-implantation of undoped polysilicon would result in dislocations of the silicon surface which would be propogated during the drive-in heat treatment step. This conclusion is supported by the literature. For example, an article by Akasaka et al., "Application of Diffusion from Implanted Polycrystalline Silicon to Bipolar Transistors", Japanese Journal of Applied Physics, Vol. 15 (1976), Supplement 15-1, pp. 49-54, describes a method for providing a P-doped base area in a silicon surface by drive-in or diffusion from a polycrystalline layer adjacent the silicon surface. In the process, the polycrystalline silicon is implanted with boron at a low concentration level below the critical dosage known to cause damage to prevent incurring any surface damage at the interface between the polysilicon and the silicon. Thereafter, a drive-in heat treatment is used to diffuse the boron to a depth of about 10,000 .ANG. to provide the base region.
There have been no reports or efforts made to produce a P-type emitter region in a PNP transistor by means of diffusion from a doped polysilicon layer, whether in-situ doped or doped by ion implantation.